This invention relates to repeaters for Ethernet or similar packet-based data communication networks, wherein at a hub or repeater a multiplicity of sources of data are provided with access by way of a contention-resolution process. The invention particularly concerns the transmission of multi-media data, such as voice or television data, in packet form, through such a repeater.
Network repeaters which allow onward transmission by one data source at a time on a common network path are well known. It is known to use a system in which a source which has data to transmit listens for the presence of carrier in the common path and transmits its data only if no contention with existing carrier is detected. In modern, digital systems, the detection of carrier is represented by the detection of, for example, non-idle data symbols on the common path and permission to transmit is accorded if transmission is to commence after some defined interval (e.g. an inter-frame gap) measured from after an end of frame delimiter field, as in for example the IEEE.802.3 standard for carrier-sense multiple access with collision detection.
It is also known, in systems of this nature, to require a station which has data to transmit and detects contention, so that it is prevented from transmitting in accordance with the relevant standards, to repeat its attempt to transmit only after a delay which is pseudo randomly selected to be equal to some multiple, between 1 and n (where n may be typically 10) of a minimum xe2x80x98back-offxe2x80x99 time. Such arrangements are quite successful and are in widespread use where the system can tolerate some variable delay (i.e. latency) in transmission of data packets from any of the stations (i.e. buffered ports) that may transmit data.
Difficulties necessarily arise when one or more of the stations has to be accorded a higher priority. One example of a scheme for enabling high priority stations and low priority stations to transmit using the same basic contention-resolution process is described in EP-A-0833479. That describes a contention-resolution process wherein high priority stations transmit in cycles and within each cycle they transmit in an order determined from their rankings, and each station which wishes to transmit further data immediately after it is transmitted data in a current cycle refrains from the contention-resolution process until it is determined that the current cycle has been completed. The aforementioned document discloses the transmission of television data via Ethernet interfaces which carry other, low priority traffic.
The present invention is concerned with the problem of giving a repeater receiving port a high priority for onward transmission of data in the event that the port is receiving multi-media traffic such as data packets conveying television data, which need transmission through a network with a substantially consistent latency. Although a variety of schemes can be devised which give a particular port high priority, the assignment of high priority to a port for all traffic through the port tends to degrade the performance of the network in respect of other traffic from other ports.
The present invention is based on a technique wherein packets received at a receiving port of a repeater are buffered and at least partially parsed or examined to determine whether they are multi-media packets. If a packet is thereby determined to be from a source or intended for a destination which is identified as a multi-media device the device may be identified in a look-up table so that subsequent packets from the same source or to the same destination may be identified as multi-media packets by reference to the look-up table. A multi-media packet identified in this manner is given priority by the modification of the contention-resolution rules in its favour. In particular, whereas according to the ordinary rules, packets cannot be transmitted if there is detection of contention and must wait for a pseudo random period before a further attempt to transmit is made, a packet which is given priority according to the invention can be the subject of a new attempt to transmit after a fixed time from the detection of contention. If the packet is not so treated, then it w ill be transmitted according normal contention resolution rules operative in the repeater, such that if contention is detected re-transmission will be attempted after a pseudo random delay.
Provided that multi-media traffic is a fairly low percentage of traffic through the repeater, for example not greater than ten percent, the technique may work well because the bulk of the signal traffic through the repeater will not be given unfair advantage. The technique is intended to improve consistency of latency for multi-media traffic not necessarily to improve the throughput of it.
The technique according to the invention may be relevant to any bridge-repeater design, where bridging is made between segments and repeating performed on each segment and it may be adapted for systems with both half and full duplex link, with buffering at a receive port only or with buffering both at transmit ports and receive ports. It may also be applied to a full bridge between a multi-media source link and a repeater.